Ventilators configured to ventilate a room have been conventionally known, and as such as ventilator, a humidity controlling ventilator configured to exhaust room air and simultaneously dehumidify outside air to supply the dehumidified air into a room has been known. PATENT DOCUMENT 1 describes a humidity controlling ventilator serving as the above-described ventilator. The humidity controlling ventilator determines the rotational speed of a fan such that the accumulated power consumed by the fan is equal to the power required to provide a previously determined target air flow rate. Specifically, in the conventional humidity controlling ventilator, the fan has been controlled based on the mass flow rate of air blown by the fan (the mass of air per unit time).